Digital Image Resolution

A confusing topic for beginners, here we look at image resolution.
Colour for digital imaging purposes is defined by a number of channelsof data, one for each colour required by the system of representationthat we are using. A monochrome image requires only one channelfor the brightness or luminance. For colour images the two mainsystems are RGB and CMYK. RGB, which stands for red, green andblue, models the way our eyes respond to colour. All computermonitors display in RGB and digital cameras and scanners actuallycapture the image in RGB, even if it is converted to somethingelse. CMYK, which stands for cyan, magenta, yellow and black, isthe colour system used by the printing industry and almost all printers(except photo printers) use four inks or dyes to print colourimages. It is possible to convert between the two, although it isbetter to do this only once.

Digital images, often known as bitmap images, have two maincharacteristics: spatial resolution and colour resolution. Alldigital images, whether they come via a scanner or digital camera,comprise discrete picture elements, called pixels . The number ofpixels that make up an image, in both the horizontal and verticaldirections, defines its spatial resolution. The resolution can beexpressed in two ways, as an absolute resolution, such as a 1280 x 1024pixel image or as a size and the number of pixels or dots per inch orcm, such as a 4″ x 6″ print at 300 dpi. The colour resolutionindicates how finely divided and accurate are the colour values. It is also sometimes called the colour depth. Typical digitalimages have 8 bits of data for each colour channel, giving 2 to thepower of 8 or 256 levels. The norm for an RGB is 8 bits of red, 8of green and 8 of blue, giving 24 bits per pixel and over 16 millionpossible colour values. More bits give more room to manipulatethe image, so many scanners and digital cameras will actually capturemore bits per channel.

What resolution do I need?

This is probably the hardest thing for a beginner to figure out. Do Ineed a 5 million pixel (5Mpixel) camera or will a 1.3Mpixel do? ShouldI print at 300dpi or 1440dpi? The answers all depend on the purpose.Let’s talk about input resolutions first, either from a camera orscanner. You need to get enough pixels for the purpose. If you areplacing images on the web, say for a personal web site or to email tosomeone, you will usually never need more than an 800 x 600 pixelimage. This is a half-mega pixel image, so you can see thatalmost any digital camera will work. Also pretty much any scanner willallow you to turn chemist shop photo prints into this resolutiondigital files.

When printing images you need to take into account two factors, thesize of the desired print and the resolution your printer needs to givea good result. Now it is very important not to be confused by aprinter’s rated resolution. Just because a printer has a resolution of,say, 2880dpi doesn’t mean this is the resolution you need to send it.Far from it. Printers put dots on the page. The printer’s resolutionreflects the size and spacing of these dots. However, each of thesedots is not the equivalent of one colour channel in your photograph.The dots a printer puts down are usually only of a single colourintensity or maybe the printer can place a limited number of differentdot sizes, so it can represent maybe eight shades of the colour. Thusto get 256 levels of cyan the printer has to put a group of dots down.Most desktop printers work quite well if you send them an image ofbetween 200 and 360dpi. So say you want a digital camera with which youcan produce prints on your inkjet printer of 8 x 10 inches. 8 x 10 at300dpi means an 2400 x 3000 pixel image, or around 7Mpixels. Nowthere aren’t many consumer level cameras that can do this. Butresolution can be interpolated (fancy word for increased throughguessing). Photoshop, for instance, allows you to resize an image andyou can add pixels to it. I have found that most digital camerapictures can be interpolated up by a factor of two with no problems,and usually by a little more. So instead of a 7Mpixel camera, one ofthe 3.3Mpixel ones will do fine. In fact if you only occasionally print8 x 10 and usually only 5 x 4 you can happily get away with a 2.1Mpixelmodel.

These three versions of the sameimage have been saved with differing numbers of bits per channel. With lowering colour resolution objectionable banding occurs in areasthat should be smoothly shaded, like the sky.